This invention relates to powdered alloys and, more particularly, to a method for alloying lithium with a powdered magnesium composition.
2. Prior Art
There has been an ongoing effort to develop high strength aluminum alloys that would be characterized by stronger, stiffer and lighter weight properties and which would be adapted for use in diverse areas such as aircraft, aerospace, automotive, naval, and electrical industries. While high strength is particularly important, the aluminum alloy must also meet a combination of property requirements such as density, strength, ductility, toughness, fatigue and corrosion resistance, with specific requirements being a function of the end use of the alloy. It is known in the art that high strength, lighter weight aluminum alloys can be obtained by alloying aluminum with lithium and that one or more additional alloying elements can be included in the alloys to provide suitable properties for particular end uses.
The general characteristics of aluminum-lithium alloys are described in the Encyclopedia of Science And Chemical Technology, 6th Ed., 1987, Vol. 1 at Page 426. In this reference, it is disclosed that the addition of lithium to aluminum provides an alloy that is characterized by low density, an increase in elastic modulus (stiffness), and an increase in strength. It is pointed out that lithium is the lightest metal in existence and that for each weight percent of lithium added to aluminum, there is a corresponding decrease of 3% (theoretical is 5%) in the alloy's weight. It is noted that as the amount of lithium in the alloy is increased, there is a corresponding increase in strength due to the presence of very small precipitates which act as strengthening agents with respect to the aluminum and that as the precipitates grow during heat treatment, the strength increases to a limit and then begins to decrease. Accordingly, it is pointed out that aluminum-lithium alloys come under the classification of precipitation-strengthening alloys and that they are also classifiable as heat-treatable because the size and distribution of the precipitates can be controlled by heat treating. Also, it is reported that the addition of lithium to aluminum results in an alloy with unacceptable (low) levels of ductility for many applications and, therefore, other elements such as copper, magnesium and zirconium have been included in the alloy to offset the loss in ductility; however, it is further reported that these alloy additions, particularly copper, increase the alloy density and, therefore, the development of alloy formulations has focused on balancing the various positive an negative attributes of the different elements, to arrive at composition with suitable properties.
It would be advantageous to provide a high strength magnesium-lithium alloy which has the beneficial properties hereinabove described for aluminum-lithium alloys, but which it substantially lighter and, therefore, more cost effective with respect to end use applications.
U.S. Pat. 3,563,730 (Bach et al., 1971) discloses, in Example 7, a method for preparing magnesium-lithium alloys in particulate form which comprises mixing a dispersion of molten lithium in mineral oil with granular or powdered magnesium at a temperature above the melting point of lithium but below the melting point of the alloy to be produced, and continuing the mixing until alloying has been effectively achieved.